greybus: bundle: remove private data field

Remove the private data field from the bundle structure as it is no
longer needed. Bundle drivers can use the driver data field in the
bundle device.

Update the only current user to use the connection private data until it
has been converted to a bundle driver.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>